In the prior art, a latent electrostatic image is developed by dry toner particles or by toner particles dispersed in an insulating nonpolar liquid. The dry toner particles cannot be too fine, since they will become airborne and be disadvantageous to health should they escape into the circumambient atmosphere. Furthermore, the dry toner particles must be fixed by fusing at elevated temperatures, which requires a source of energy. The developing of latent electrostatic images by dry toners results in images which do not have the degree of resolution which is desirable. Liquid-carried toners, however, may be as fine as one can make them, since there is no danger of their becoming airborne. Accordingly, they may be employed to produce copy of increased resolution.
An electrostatic image may be produced by providing a photoconductive layer with a uniform electrostatic charge and thereafter discharging the electrostatic charge by exposing it to a modulate beam of radiant energy. It will be understood that other methods may be employed to form an electrostatic image, such, for example, as providing a carrier with a dielectric surface and transferring a preformed electrostatic charge to the surface. The charge may be formed from an array of styluses.
This invention will be described in respect of office copiers, though it is to be understood that it is applicable to other uses involving electrophotography.
In an office copier, after the latent electrostatic image has been formed, usually by projecting the desired information upon a charged photoconductor in the dark, the image is developed by a liquid comprising pigmented toner particles dispersed in a nonpolar, nontoxic liquid having a high-volume resistivity in excess of 109 ohm centimeters, a low dielectric constant below 3.0, and a high vapor pressure. Suitable liquids, acting as dispersants, are the aliphatic isomerized hydrocarbons prepared by the Exxon Corporation and sold under such trademarks as ISOPAR-G, ISOPAR-H, ISOPAR-L and ISOPAR-M, each having different distillation points and vapor pressures.
After the image has been developed, it is transferred to a carrier sheet. During transfer, there occurs a degree of smudging, smearing, or squashing of the image. This reduces the resolution. Furthermore, the entire image does not transfer from the photoconductor to the carrier sheet. This leaves a residue of toner on the photoconductor which formed the image just transferred. The squash effect may be avoided by providing a gap between the developed image on the photoconductor and the carrier sheet to which the image is to be transferred. The density of the image and the resolution of the gap-transfer method are good, but are improved by the present invention.